It is generally highly desirable to include all elements of an electronic design for mass production into as few integrated circuits as possible. The benefits of such integration are well known and numerous, including, for example, lower parts cost, smaller size, lower manufacturing cost for a final product, higher reliability and greater function. Such benefits are realized whether the function to be integrated is primarily digital or of an analog nature.
Unfortunately, the integration into integrated circuits, ICs, of many analog functions, for example filters, analog timers, FM detectors and the like, suffers in practice from a well known deficiency of most integrated circuit manufacturing processes. Basic analog components, for example resistors and capacitors, of sufficient accuracy for many analog design purposes, are not generally manufacturable within an integrated circuit. More specifically, many such components typically have a tolerance of plus or minus 30 percent from their nominal values due to manufacturing process variation when rendered in an integrated circuit.
There have been numerous approaches employed in the prior art to overcome such limitations. For example, it is common to use accurate off-chip components, such as resistors and capacitors, in designs requiring accurate values for such components. The off-chip components typically may have much greater accuracy than on-chip components. For example, discrete resistors with one percent tolerance are commonly available, though they cost more than “standard” components, e.g., components with tolerances of plus or minus 10 percent. Although integrated circuit designers strive to require few off-chip components, those that are required cost more than the equivalent implementation on an IC, require additional manufacturing steps and increase the size of a final product. In addition, external components and the additional manufacturing process steps required to place them in an assembly reduce reliability and manufacturing yield.
Another common prior art approach to overcome the accuracy limitations of on-chip components is to utilize off-chip adjustable components. Such adjustable components are commonly used in radio frequency designs. The use of off-chip adjustable components is even less desirable than using off-chip discrete components, because adjustable components tend to be larger and require an additional manufacturing process step, and frequently expensive manufacturing test equipment, to make the adjustment. Never-the-less, off-chip adjustable components are widely used in conjunction with integrated circuits in electronic assemblies.
Yet another well known approach to overcome the accuracy limitations of on-chip components is illustrated in FIG. 1A (conventional art). The circuit 100 depicted in FIG. 1A uses a phase locked loop (PLL) to set filter frequencies, with the filter frequencies based on values of on-chip resistors and capacitors.
The frequency of the voltage-controlled oscillator (VCO) 110 is proportional to the transconductance, gm. The nominal value of gm, and hence the nominal VCO frequency is set by on-chip resistance. The PLL acts to adjust the on-chip resistance such that the product of R times C matches the desired value, e.g., an RC constant that is proportional to the crystal oscillator frequency. The PLL locks when the frequency of the VCO equals the crystal oscillator frequency. The control voltage 120 for the VCO is then used to set the gm value of other circuits, e.g. filter 130, on the IC in order to set their frequencies. Since the crystal oscillator provides a very accurate frequency (based on physical properties of the crystal), the variation of the on-chip RC value is corrected.
Unfortunately, this approach has several drawbacks. First, this method requires the design of a complicated PLL system. It will produce spurious signals from the phase detector, which can introduce deleterious signals into subsequent circuits. In addition, it has undesirable power consumption as it must be active at all times. Further, this circuit requires variable transconductance elements and is not widely applicable to a variety of applications, such as op-amp based filters. This method also requires a voltage controlled oscillator whose output signals can interfere with sensitive on-chip circuitry.
Still another well known approach to overcome the accuracy limitations of n-chip components is illustrated in FIG. 1B (conventional art). The circuit 150 depicted in FIG. 1B uses a frequency locked loop (FLL) to set filter frequencies, with the filter frequencies based on values of on-chip resistors and capacitors.
On-chip resistance sets the nominal value of gm. The FLL acts to adjust that resistance such that the product of R times C matches the desired value, e.g., an RC constant that is proportional to the crystal oscillator frequency. The FLL locks when the 90 degree phase shifter center frequency equals the crystal oscillator frequency. The control voltage 170 for the phase shifter is then used to set the gm value of other circuits, e.g., filter 180, on the IC in order to set their frequencies. Since the crystal oscillator provides a very accurate frequency (based on physical properties of the crystal), the variation of the on-chip RC value is corrected.
Unfortunately, this approach also has several drawbacks, similar to the previously described PLL circuit. First, this method requires the design of a complicated FLL system. It will produce spurious signals from the phase detector, which can introduce deleterious signals into subsequent circuits. In addition, it has undesirable power consumption as it must be active at all times. Further, this circuit requires variable transconductance elements and is not widely applicable to a variety of applications, such as op-amp based filters.
An additional well known approach to overcome the accuracy limitations of on-chip components is to adjust on-chip components for accuracy, for example by laser-trimming them. Adjustment processes such as laser trimming can produce highly accurate components, but the process is generally complex, time-consuming and expensive, requiring additional manufacturing process steps and expensive calibration and processing tools. The integrated circuit itself is also typically larger to allow for adjustable structures and room for the adjustments themselves, resulting in a higher cost for the bare integrated circuit with adjustable analog components, in addition to the increased manufacturing costs due to the adjustment itself.
Unfortunately, for a substantial class of applications, for example radio frequency filters, analog timers, FM detectors and the like, existing approaches to overcome the known large tolerance problems with analog component values within integrated circuits are not commercially acceptable to many IC manufacturers.